1. Field of the Invention
The present invention relates to an optical information recording and reproducing method and apparatus for reproducing information utilizing maximum-likelihood decode, and in particular to an optical information recording and reproducing method and apparatus for adjusting various parameters for recording and reproduction using evaluation indexes.
2. Related Background Art
Conventionally, an error rate of a reproducing signal is used when adjustment of a recording laser power at the time of recording, a reproducing laser power at the time of reproduction, tracking control, and the like is performed in an optical information recording and reproducing apparatus. This is for recording or reproducing a signal while setting the above-described various parameters to a plurality of predetermined adjustment values and using an adjustment value with a minimum error rate as an optimum value in subsequent recording and reproduction.
However, there is a problem in that a larger number of samples are required and time required for adjustment is extended when an error rate is low in the case in which adjustment values are searched using an error rate as an evaluation index.
In addition, it is disclosed in the literature “A Window-Margin-Like Procedure for Evaluating PRML Channel Performance”, IEEE Trans. On Magnetics, Vol. 31, No. 2, March 1995 that a Sequenced Amplitude Margin (SAM) is used as an evaluation index.
This is for using a likelihood of Viterbi decode as an evaluation index. In using the SAM, a likelihood (also called as path metric) of the most sure reproducing signal sequence and a likelihood of a secondly more sure reproducing signal sequence are found at each time to evaluate a likelihood difference.
However, in the case in which a main factor of occurrence of an error in a reproducing signal is an edge portion, in the SAM, there is a problem that a detection sensitivity declines because differences of likelihoods obtained by all signal sequences are used regardless of the edge portion of the reproducing signal, and as a result, an accuracy of an adjusted value falls.
In addition, in the conventional example disclosed in Japanese Patent Application Laid-Open No. 10-21651, a likelihood difference between signal sequences of a minimum Euclidean distance is used targeting only an error of the minimum Euclidean distance. However, an error is not limited to that of the minimum Euclidean distance, and generation of an error depends upon a recording and reproducing system and design parameters such as a partial response characteristic of Partial-Response Maximum-Likelihood (PRML). In addition, since it is necessary to detect a path of the minimum Euclidean distance, an apparatus becomes complicated.
As described above, in the conventionally known method, it is difficult to realize highly accurate parameter adjustment with small number of samples and a simple structure.
In addition, when a mark optically recorded in a recording medium is reproduced, since there is shift (advance and delay) of an edge portion of a signal called jitter and this edge shift becomes a factor of occurrence of an error, it is difficult to increase an adjustment accuracy with a small number of samples.